Hyponatremia after initiation and rechallenge with trimethoprim–sulfamethoxazole in an older adult

PURPOSE: The purpose of this study is to describe a case report of a patient experiencing hyponatremia from trimethoprim-sulfamethoxazole (TMP-SMX) upon initial use and subsequent rechallenge. SUMMARY: An 82-year-old woman presented to the emergency department with altered mental status thought to be due to complicated cystitis and was treated with TMP-SMX 160 mg/800 mg orally twice daily for 7 days. Her basic metabolic panel prior to initiation of TMP-SMX was within normal limits, with the exception of her serum sodium of 132 mmol/L (range 133-145 mmol/L). The day after completing her 7-day course of TMP-SMX therapy the patient was evaluated by her primary care provider and another basic metabolic panel revealed a reduction in the serum sodium to 121 mmol/L. The patient's serum sodium concentrations increased to baseline 7 days after completion of the TMP-SMX therapy, and remained normal until she was treated in the emergency department several months later for another presumed urinary tract infection. She was again started on TMP-SMX therapy empirically, and within several days her serum sodium concentrations decreased from 138 mmol/L to a low of 129 mmol/L. The TMP-SMX therapy was discontinued upon negative urine culture results and her serum sodium increased to 134 mmol/L upon discharge. Based upon the Naranjo probability scale score of 9, TMP-SMX was the probable cause of the patient's hyponatremia. CONCLUSION: Our patient developed hyponatremia from TMP-SMX therapy upon initial use and rechallenge. Although hyponatremia appears to be rare with TMP-SMX therapy, providers should be aware of this potentially life-threatening adverse event.